The field of the invention is cavity ring down spectroscopy and more specifically cavity ring down spectroscopy for strongly absorbing species.
Cavity Ring Down Spectroscopy (CRDS) is a laser absorption spectroscopy technique used to measure absolute concentrations of absorbing species. In a conventional CRDS system, a sample chamber containing an absorbing material is placed in an optical resonator consisting of two spherical mirrors facing each other along a common optical axis. Light incident on and passing through one mirror circulates back and forth multiple times in the resonator, setting up standing waves having periodic spatial variations. Light exiting through the other mirror is measured to determine the light intensity present in the cavity
FIG. 1 shows a prior art CRDS system. The sample absorbing material is placed in chamber 102 and two spherical mirrors facing each other are shown at 100 and 104, with the entire system stabilized on an optical table, partially shown at 101 in FIG. 1. An optical signal such as a laser beam 106 is introduced by laser 103 into the ultra-low-loss cavity and then turned off. The light introduced through mirror 104 circulates back and forth multiple times between mirrors 104 and 100. As the light oscillates inside the cavity, a small fraction is transmitted out of the cavity at each mirror. The optical photodetector shown at 105 measures the light intensity exiting the cavity. The intensity of this light exiting the cavity decays exponentially in time at a rate dependent on the total cavity losses. These losses comprise the loss from the finite reflectivity of the mirrors, from diffraction, and from any absorption within the cavity. The radiant energy stored in the resonator decreases in time, a process referred to as xe2x80x9cring down.xe2x80x9d
A graphical representation of this ring down phenomena is shown in FIG. 2. In FIG. 2, light intensity is represented on the y-axis at 200 and time is represented on the x-axis at 201. The decay of light intensity due to sample species absorption is shown by the curve at 202. The exponential decay of the light intensity is mathematically defined as I(t)=exe2x88x92t/xcfx84, where xcfx84 is the time constant that characterizes the exponential decay.
For an empty cavity that does not contain an absorbing sample, the stored radiant energy follows an exponential decay characterized by a ring down time constant that depends only on the reflectivity of the mirrors and the speed of light in the resonator. When a sample is placed in the resonator the ring down is accelerated and ideally, the intracavity energy decays almost perfectly exponentially. It can be shown that comparison of the decay time constants measured for an empty cavity and one in which absorbing species are present can be used to quantitatively determine the absolute concentration of absorbing species present in the cavity (at the given input wavelength). An absorption spectrum for the sample is obtained by plotting the reciprocal of the ring down time constant versus the wavelength of incident light. These measurements, when combined with sample specific information, are used to determine the absolute concentration of the absorbing species.
It should be noted that the sample material concentration information is contained in the ring down decay time and not the initial intensity of the laser light. Therefore, this technique is insensitive to intensity variations in the input laser. This fact enables CRDS to produce absolute concentration measurements and sets CRDS apart from competing absorption spectroscopy techniques, which can only provide data on relative concentrations. Additionally, CRDS can measure ultra-low concentrations of absorbing species (less than 1 part per billion).
CRDS has been used to characterize a variety of gas environments and in thin films deposition through evanescent coupling of the optical wave inside a totally internally reflecting prism. CRDS has also been used for measuring the concentration of contaminants in pollution control systems.
Currently, CRDS requires the total round trip losses of the cavity to be small (much less than a fraction of a percent) if useful ring down time constants are to be obtained. Therefore, the technique is currently limited to the characterization of samples (typically gas phase species) with low absorption coefficients. This precludes the characterization of liquids and solids using CRDS, as their high absorption coefficients and significant background losses result in ring down times too short to measure.
The present invention provides an improved capability cavity ring down spectroscopy (CRDS) system and makes the CRDS techniques applicable for samples with high absorption coefficients. The present invention uses the optical gain available in a fiber amplifier to compensate for all system optical losses commonly seen in conventional CRDS systems, plus those due to fiber coupling and light detection. Additionally the present invention provides a CRDS system that is immune to cavity misalignment and therefore eminently capable of portability.
The present invention provides a cavity ring-down spectroscopy device and method suitable for materials with low and high absorption coefficients. An optical signal introduced into an optical cavity resonates through a length of optical fiber coupled to the optical cavity. The optical signal resonates through a fiber amplifier active section resulting in a gain, which compensates for system optical losses. The gain obtained by use of the fiber amplifier is then modulated between two predetermined levels. By virtue of employing a fiber amplifier and avoiding the alignment sensitivity of optical mirrors, the cavity ring down spectroscopy device and method of the present invention are insensitive to misalignment and therefore capable of portability.
It is therefore an object of the invention to provide a cavity ring down spectroscopy system with the ability to measure absolute concentrations of absorbing samples characterized by significant losses, such as liquid or solids.
It is another object of the invention to provide a cavity ring down spectroscopy system insensitive to optical misalignment.
It is another object of the invention to provide a cavity ring down spectroscopy system and method where light resonates through a length of fiber amplifier coupled to an optical cavity.
It is another object of the invention to provide a cavity ring down spectroscopy system and method where light resonates through a length of fiber amplifier coupled to an optical cavity resulting in minimal loss.